The transit of Venus across the face of the Sun has started as thousands of people across Britain are expected to watch the rare moment.

At 6.19am, the Earth's closest planetary neighbour began to cut a diagonal path across the bottom of the Sun. Appearing as a black disc 30 times smaller than the Sun's diameter, it will slowly move from left to right over the course of six hours.

The event, which ends at 12.23pm, does not occur again until June 6, 2012 - but on that occasion will not be visible from British skies. Britons will have to wait until 2247 to see Venus make another journey across the Sun.

Sunny skies should ensure that in most areas, at least, it will not be necessary to wait another 243 years for a chance to see Venus cross the face of the Sun.

It is the first Venus transit since 1882 and the last time it was observable from the UK was in 1283. On that occasion, long before the invention of the telescope, the phenomenon went unnoticed.

A Venus transit is similar to a solar eclipse, except that it is the planet Venus rather than the moon that passes between the Sun and the Earth. Weather forecasters say the chances of an unobscured view of the transit are good for central, eastern and southern England.

Clouds could block out the Sun in Scotland, Northern Ireland, and the western fringe of Wales and England.